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50 The PCB Design Magazine • August 2016 Dangling via stubs can distort signals pass- ing through your interconnect, and decrease the usable bandwidth of the signal. A via stub acts as a transmission line antenna, and has a resonant frequency determined by the quarter wavelength of the structure. At this frequency, the transmitted signal is greatly attenuated, by up to 3dB. For low-frequency signals, this is not much of an issue because these signals are sig- nificantly lower than the resonant frequency of the via stub. However, for higher-frequency signals (>1GHz), which are becoming more common as performance specifications are increased, this is- sue becomes a problem because the signals are transmitted at frequencies near or at the reso- nant frequency of the via stub. Harmonic com- ponents that are odd multiples of the funda- mental frequency can also be highly attenuated. The conventional solution to this problem is to back-drill (or control-depth drill) the vias to bore out the via stub barrels, so that the via stubs are reduced in length, if not completely removed (Figure 1). If the via is short, compared to the signal rise time, then it acts mostly as excess shunt capaci- tance. The entire length of the via contributes to the capacitance, while only the section where the signal current actually flows makes up the inductance. However, a long via stub can devel- op resonance that exacerbates the effects of its capacitance. I should point out that it is fine to have a plated through-hole (PTH) via, provid- ing the signal goes in at one end and out at the other, using the entire length of the barrel. When a via's stub length is equal to a quarter wavelength of the signal frequency, the signal travels from the trace to the end of the stub and by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA How to Handle the Dreaded Danglers, Part 1 BEYOND DESIGN Figure 1: The unterminated via stub (left) and a back-drilled via stub (right).

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